The present invention relates to high-intensity discharge lamps and, more particularly, to metal halide lamps. With such lamps, light is emitted primarily from vaporized metal atoms excited by electron collisions in the arc discharge path. The metal atoms are introduced into the lamp in the form of metal halides. They are vaporized after starting by the hot arc discharge plasma which typically comprise ionized rare gases and mercury. The metal halides have much higher vapor pressures than elemental metals and, therefore, produce a far greater concentration of metal excited states in the discharge arc, which greatly enhances radiation output. Using metal halides, a wide variety of metal atoms can be introduced into a lamp, each with unique emissions which can be combined to create desired spectra. In this manner, lamps of high efficacy can be made which emit visible light sufficiently white to be pleasing to the human eye and useful for general illumination purposes. Lamps emitting nonvisible ultraviolet radiation can also be made for special applications, such as semiconductor processing or germicidal irradiation. The color of a visible light source is typically described in terms of Correlated Color Temperature (CCT), Color Rendering Index (CRI), and Chromaticity (expressed in x and y coordinates).
In designing a metal halide lamp, a combination of metal halides is chosen to achieve a desired color at a designated power. Deviations from the designated rated power typically result in changes in color. In particular, reducing power usually results in a color which is less xe2x80x9cwhitexe2x80x9d than desired. The present invention provides a method of adjusting the color of a metal halide lamp by enhancing the radiation of specific metal atoms. In one embodiment, the invention can be used to maintain a constant color temperature (CCT) during dimming.
This invention most conveniently can be realized by the utilization of an electronic ballast as described in xe2x80x9cA New Electronic Ballast for HID Lampsxe2x80x9d, Nishimura et al., Journal of IES, Summer 1998, Page 70; and xe2x80x9cHigh Frequency Discharge Lamps on High Frequency Powerxe2x80x9d, J. H. Campbell, Journal of IES, Dec. 1969, Page 713. In this particular ballast, the waveform is typically a square wave of relatively low frequency. The low-frequency operation, that is, between 50 and 400 Hz, has been shown to be very successful, and these ballasts are widely available in the marketplace. Examples of such ballasts are made by Aromat Corp. (NAIS brand), WPI Corp. and Phillips. We have combined the operation of such an electronic ballast with the metal halide lamp so we are able to improve some of the characteristics of the light source which will be described further herein.
As mentioned heretofore, experiments have indicated that there are few practical ways of changing the color temperature of a metal halide lamp at rated power. We have found that under dimming conditions, the CCT and the hue change involuntarily, and in an undesirable manner. If the same CCT level and illumination are maintained, changing the CCT in a controlled manner cannot be accomplished with existing metal halide lamps and normal ballasting conditions. Most ballasts used today are magnetic ballasts and there is no controlled way of changing the CCT at rated power.
Lamp-to-lamp variations in CCT are an annoying characteristic of currently available metal halide lamps. Due to manufacturing process tolerances, the fills and arc tube geometry of lamps are not exactly the same from lamp to lamp. This results in lamp-to-lamp color variation of as much as xc2x1400xc2x0 K. in quartz lamps and xc2x1200xc2x0 K. in ceramic metal halide lamps. This slight appearance difference from lamp to lamp is perceptible and annoying, especially when lamps are next to each other in a typical ceiling/downlight application. This color temperature variation can be perceived especially in color-sensitive applications, such as illumination used for fabric, fresh produce or art, for example.
A further problem occurs even if lamps are made identically and they all have the same color hue at 100 hr. Due to different aging circumstances, the color/hue tends to change somewhat differently in each lamp. This is perceptible to the eye after lamps have burned about 2000-4000 hrs. The concept of altering the duty cycle in lamps has been investigated in fluorescent lamps primarily for reducing the power into the lamp (i.e., dimming) (xe2x80x9cDesign Considerations for Optimum Ignition and Dimming of Fluorescent Lamps Using a Resonant Inverter Operating Open Loopxe2x80x9d, Ribas et al., Proceedings of IEEE/IAS Conference, 1998, Page 2068, Vol. 3.) In all cases, however, the waveform is symmetric, and in many cases sinusoidal.
Therefore, an object of the present invention is to provide a light source where at rated power the color can be varied depending on the application either for mood control or for special effects, such as museums or color-sensitive merchandise to accentuate certain colors. The ability to change the color at will and in a controlled manner is a highly desirable feature.
Another object of the present invention is to ameliorate the problem of lamp-to-lamp color variation in metal halide lamps, especially in low wattage metal halide lamps, both initially and throughout the life of the lamp.
Another object of the present invention is to ameliorate the problem of lamp color change over its life. Typically, lamps can have a particular color temperature, but after burning several thousand hours they tend to change colors, as mentioned above.
A further object of the present invention is to be able to burn the lamps and make sure that the color does not change during dimming. Often times, primarily for energy saving purposes, dimming is highly desirable. However, the color changes substantially during dimming in an uncontrolled and undesirable manner. Thus, it is highly desirable to maintain the design point color temperature of the lamps during dimming.
Another object of the present invention is to provide a metal halide lamp that has a CRI that does not deteriorate during life and is substantially the same from lamp to lamp.